1. Field of the Invention
The present invention relates to a liquid crystal display device for use in devices such as display apparatus for displaying text and figures and the like, dimmer devices by which the degree of transmittance of incident light is varied, and optical shutters.
2. Description of the Related Art
A display device which utilizes the optical switching phenomenon of a ferroelectric liquid crystal (FLC) has been proposed by N. A. Clark and S. T. Lagerwell (App. Phys. Lett., Vol. 36, p899 (1980)) as a liquid crystal display which can offer a wide viewing angle and a fast response. However, the display device (a surface stabilized ferroelectric liquid crystal (SSFLC) optical device) is bistable, and suffers from the problem that generating a gradation display by controlling the voltage is difficult. To overcome this problem, several methods have been proposed for using an FLC to realize a gradation display. One such example is Japanese Unexamined Patent Application, First Publication No. Hei 6-194693, in which a technique for adding fine particles to a liquid crystal is disclosed. By distributing the fine particles uniformly though the liquid crystal, a dielectric constant distribution is formed, and as a result, a distribution is formed for the effective voltage applied to the liquid crystal. The effective voltage distribution makes a gradation display possible. Furthermore, Japanese Unexamined Patent Application, First Publication No. Hei 9-236830 a technique is disclosed for forming a striped structure within a liquid crystal by reaction with a monofunctional monomer. By providing threshold characteristics which differ for different local regions within the liquid crystal, the striped structure controls the domain surface area generated on voltage application, and therefore makes a gradation display possible. However the aforementioned techniques suffer from problems such as the generation of orientation defects, which is linked to a reduction in contrast resulting from the introduction of foreign material into the liquid crystal, and the suppression of increases in driving voltage.
An alternative liquid crystal material which has an antiferroelectric phase has been reported by Chandani et al. (Jpn. J. Appl. Phys., 28 (1989), L1265), and a display device utilizing an antiferroelectric liquid crystal (AFLC) has also been proposed (Jpn. J. Appl. Phys., 29 (1990), 1757). AFLC materials have tristability based on a phase transition between an antiferroelectric phase and a ferroelectric phase, and by switching between phases under a bias voltage application, a display device can be manufactured in which gradation display can be achieved by voltage control. However, this type of device also suffers from problems such as the necessity for a bias voltage to achieve a gradation display, and the fact that the driving waveform for a high precision display device with a large number of scanning lines is very complex.
In contrast, Inui et al. and Tanaka et al. have reported an AFLC material (hereafter termed a non-threshold AFLC material) for which the curve displaying optical transmittance relative to applied voltage is a V shape (Proceedings of the 21st Liquid Crystal Symposium 2C04, p.222 (1995), and p.250 (1995)). This V shaped characteristic refers to a property wherein, as shown in FIG. 1, application of a positive voltage produces a continuous variation in the transmittance, and application of a negative voltage also produces a continuous variation in the transmittance with the shape of the curve being substantially symmetrical with the curve representing application of a positive voltage across an axis at a voltage of zero volts. A non-threshold AFLC device utilizing this type of material is reported as having no clear threshold for the phase transition, and displaying a small hysteresis characteristic.
Moreover, Takei et al. have reported a 5.5 inch diagonal liquid crystal optical device which combines the above non-threshold AFLC device with a thin film transistor (TFT) (papers presented at the 74th Workshop of The Japan Society for the Promotion of Science, xe2x80x9c142nd Committee of Organic Materials for Information Displayxe2x80x9d ,Section A (liquid crystal materials), p14, 1999).
Typically, non-threshold AFLC materials display high spontaneous polarization values of 100 (nC/cm2) or more, and in the aforementioned liquid crystal optical device which combines a non-threshold AFLC device with a TFT (papers presented at the 74th Workshop of The Japan Society for the Promotion of Science, xe2x80x9c142nd Committee of Organic Materials for Information Displayxe2x80x9d , Section A (liquid crystal materials), p14, 1999), a non-threshold AFLC material with a spontaneous polarization of 229 (nC/cm2) was used. In order to drive this type of non-threshold AFLC material with a high spontaneous polarization value, an electrical charge in proportion to the spontaneous polarization needs to be injected. However, a problem arises in that because there is a limit on the amount of charge that can be supplied from a TFT, charge injections which span several frames become necessary, which slows down the screen display on the liquid crystal display device.
One example of a method for resolving the above problem comprises the addition of a large auxiliary capacitance to a TFT. However, with a large auxiliary capacitance, the numerical aperture of the liquid crystal optical device decreases, meaning the display will darken. Moreover, investigations by the inventors have revealed that as the auxiliary capacitance is increased the RC time constant also increases, and that in order to carry out sufficient writing within a predetermined writing time period, the on-state resistance of the TFT must be lowered and the TFT characteristics improved. Hence, because the capacity value increases, if the on-state current of the TFT can not be sufficiently ensured, then the writing may not be completed within the writing time period. Consequently, if the TFT characteristics are determined, and the spontaneous polarization value of the liquid crystal material and the panel structure are also determined, then a threshold value will exist for the optimum auxiliary capacitance, and an auxiliary capacitance in excess of this threshold value will cause an increase in the RC time constant, a reduction in the injected charge within the writing time period, and as a result a reduction in the charge for writing to the liquid crystal.
In order to enable the driving of a liquid crystal with a high spontaneous polarization, either a TFT with the required characteristics can be used, or alternatively writing can be conducted at a high voltage. However, in such cases the following types of problems arise. Firstly, there is a need to develop new TFTs with suitable characteristics. Secondly, drive circuits which enable the application of a high driving voltage also need to be developed. Even if these two criteria are met, a large charge still needs to be used in order to drive a liquid crystal with a high spontaneous polarization, and consequently the power consumption will be extremely large.
The various problems described above resulting from high spontaneous polarization values are not limited to gradation display devices which use non-threshold AFLC materials, and are also an issue for gradation display devices which use an aforementioned FLC, and AFLC gradation display devices which utilize tristability.
In addition, for AFLC gradation display devices which utilize tristability, it has been shown that the spontaneous polarization needs to be lowered in order to minimize the hysteresis distortion. For example in Japanese Unexamined Patent Application, First Publication No. Hei 10-279534, in order to reduce the hysteresis distortion for an AFLC display device which utilizes tristability, the spontaneous polarization was reduced by using an antiferroelectric composition which incorporates a new racemic compound. However, the spontaneous polarization value using this method was reported in a working example as approximately 116 (nC/cm2), a value which is not low enough. Furthermore, lowering the spontaneous polarization of the liquid crystal material is also being investigated as a way of suppressing a large driving voltage and large power consumption. For example in Japanese Unexamined Patent Application, First Publication No. Hei-9-151375, an antiferroelectric liquid crystal composition which displays a spontaneous polarization value at 30xc2x0 C. of between 10 (nC/cm2) and 150 (nC/2), an antiferroelectric liquid crystal composition which incorporates between 1.0 (wt %) and 90 (wt %) of a racemic modification of a compound which if optically resolved and made optically active could function as an antiferroelectric liquid crystal compound, and the aforementioned racemic compound, are disclosed for an AFLC display device which utilizes tristability. However, the basis for the prescribed spontaneous polarization value, namely between 10 (nC/cm2) and 150 (nC/cm2), is not disclosed. Furthermore, in Japanese Unexamined Patent Application, First Publication No. Hei-10-195443, an antiferroelectric liquid crystal composition is reported which is formed by mixing a liquid crystal composition having a positive (or negative) spontaneous polarization, and which comprises as the main constituents at least two positive (or negative) spontaneous polarization compounds with different optically active groups, together with a racemic modification of a liquid crystal compound having the same optically active groups as the liquid crystal compounds above. Furthermore, in the case where the liquid crystal composition has a positive (or alternatively negative) spontaneous polarization, the absolute value of the spontaneous polarization is small for a liquid crystal composition formed by further combining at least one compound which has a positive (or alternatively negative) spontaneous polarization and which moreover has an optically active group different from the optically active group of the aforementioned liquid crystal compound. Moreover, the optically active groups of the liquid crystal compounds, and the fact that the absolute value of the spontaneous polarization was less than 135 (nC/cm2) at 30xc2x0 C. were also disclosed. However even in this case, as in the example described above, no basis for the prescribed spontaneous polarization value is reported. Moreover, in both this example, as well as in the method described in Japanese Unexamined Patent Application, First Publication No. Hei 9-151375, the type of compounds which can be used are severely restricted by a number of factors. Consequently, the chemical structures of the racemic modifications which can be used in Japanese Unexamined Patent Application, First Publication No. Hei 9-151375, and the optically active groups which can be mixed in Japanese Unexamined Patent Application, First Publication No. Hei 10-195443, are limited. Unfortunately, these limitations force limitations on the characteristics of the actual liquid crystal composition itself. For example, the operational temperature range or usable temperature range for a liquid crystal composition is determined by the operational temperature range or the usable temperature range, or a combination thereof, of the compounds which make up the composition. However, in those cases where either the chemical structure of, or the optically active groups of, the compounds are limited, then the expansion of the operational temperature range or usable temperature range for the liquid crystal composition, or the setting of the temperature range to a specific temperature range becomes difficult. Moreover, this argument is also true for the tilt angle which has a significant effect on the transmittance and the contrast of the liquid crystal display device. All the above techniques relate to AFLC materials for use in AFLC display devices which utilize tristability, but as is outlined above, all have drawbacks. Furthermore, in the case of materials used for non-threshold AFLC display devices, very little is known.
An object of the present invention is to provide a liquid crystal display device with a high degree of light utilization efficiency (a high numerical aperture), and for which a rapid response (a short response time) is possible at high contrast.
A liquid crystal display device of the present invention incorporates at least one compound with optical activity for which the value of the spontaneous polarization is positive, and at least one compound with optical activity for which the value of the spontaneous polarization is negative, wherein the liquid crystal composition, for which the overall spontaneous polarization may be either positive or negative, is held between substrates which incorporate electrodes. Furthermore, another liquid crystal display device of the present invention incorporates at least one compound with optical activity for which the value of the spontaneous polarization is either positive or negative, and at least one racemic modification, wherein the liquid crystal composition, for which the overall spontaneous polarization may be either positive or negative, is held between substrates which incorporate electrodes. In this case, the ratio of R configurations relative to S configurations within the compound with optical activity may be either RS or R less than S.
Moreover in the aforementioned liquid crystal display device, the transmittance or the reflectance of the device will vary continuously relative to the applied electric field or voltage. The curve of the applied voltage relative to the transmittance (reflectance) may vary continuously only for a positive or a negative voltage, as shown in FIG. 2, or may be a V shaped curve as shown in FIG. 1. Furthermore, it is also possible that the transmittance (reflectance) is greatest when no voltage is applied, as shown in FIG. 3.
Suitable active elements for use in a liquid crystal display device of the present invention include thin film transistor (TFT) elements, metal-insulator-metal (MIM) elements, and DRAM formed from single crystal silicon. However, the invention is not limited to an active matrix drive, and depending on the ultimate use of the device, driving can also be performed with a simple matrix.
By using the present invention, a high precision liquid crystal display device is achievable which offers a high degree of light utilization efficiency (numerical aperture), high levels of contrast, and a short response time, as well as a wide viewing angle and the ability to display animation.